The present invention relates to an electron gun and, more particularly, to an electron gun for a color picture tube.
In general, a color picture tube comprises a glass envelope 11, an electron gun 12 sealed within the glass envelope 11, a deflection system 14 disposed outside the glass envelope 11, a shadow mask 15 disposed in the glass envelope 11 and having a plurality of apertures, and a phosphor screen 16 formed on the inner surface of the glass substrate 11 and opposing the shadow mask, as shown in FIG. 1. A plurality of electron beams (e.g., three electron beams) emitted from the electron gun 12 within the color picture tube are deflected by the deflection system 14. The deflected electron beams pass through the apertures of the shadow mask 15 and are incident on the phosphor screen 16, thereby forming a color image on the screen.
An "in-line type" electron gun, wherein three electron guns are aligned in a line, is generally used as the electron gun 12.
A self convergence deflection system is generally used as the deflection system 14, wherein an inhomogeneous magnetic field which has a strong pincushion-like vertical deflection magnetic field and a strong barrel-like horizontal deflection magnetic field may be formed to converge the three electron beams at the peripheral portion within the screen. When the electron beam passes through such a deflection magnetic field, the electron beam is subject to a distortion called deflection defocusing under the influence of the deflection magnetic field. As a result, at the peripheral portion within the screen, the shape of the electron beam spot is greatly distorted, as shown in FIG. 2. In other words, the beam spots at edge portions 21 along the horizontal axis become horizontally elongated to have elliptical shapes. The beam spots at four corners corresponding to edge portions 22 along the diagonal axes are formed as combinations of horizontally elongated spots 23 and vertically elongated halo portions 24, respectively. For this reason, resolution is degraded at the peripheral portion of the screen, and uniform focusing is impaired. The uniformity of focusing is degraded when a deflection angle of the picture tube is increased to within a range between 100.degree. and 110.degree.. This nonuniform focusing cannot be neglected and presents a problem.
The typical structure of a conventional bi-potential type electron gun will be described with reference to FIG. 3. The bi-potential type electron gun comprises a cathode 30, a first grid 31, a second grid 32, a third grid 33, and a fourth grid 34 which are aligned along a central axis 35. Among these elements, the cathode 30, the first grid 31 and the second grid 32 constitute a triode. The third grid 33 and the fourth grid 34 form a main electron lens 36, thereby constituting a main lens part. For example, voltages of about 150 V, 600 V, 5 kV, and 25 kV are applied to the cathode 30, the second grid 32, the third grid 33 and the fourth grid 34, respectively. The first grid 31 is grounded. The triode constituted by the cathode 30, the first grid 31 and the second grid 32 emits electron beams and forms an object for the main electron lens 36 constituted by the third grid 33 and the fourth grid 34. The electron beam is focused by the main electron lens 36 to form an electron beam spot on the phosphor screen. The second grid 32 and the third grid 33 form a pre-focusing lens 37. The pre-focusing lens 37 focuses the electron beam so as to allow the beam to be incident on the main electron lens 36.
In the electron lens as described above, it has been proposed to change the shape of the beam spot by making the shape of the openings of the grids asymmetric. For example, in U.S. Pat. No. 3,919,583, an electron gun is disclosed wherein first and second grids have vertically elongated openings, respectively. In particular, in this electron gun, the grids which constitute the triode alone have asymmetric openings. By using such first and second grids, the beam spot on the screen is extremely elongated along the vertical axis. It should be noted that the first and second grids are employed in a beam index color cathode-ray tube. This is because the beam index color cathode-ray tube generally requires an extremely vertically elongated beam spot due to its mode of operation.
Another electron gun which resembles the electron gun described above is disclosed in U.S. Pat. No. 4,322,655. In this example, a first grid has a vertically elongated opening, and the grids of a main lens part respectively have asymmetric openings. The first grid and the grids of the main lens part in this prior art example are also used in a beam index color cathode-ray tube so as to obtain a vertically elongated beam spot.
In still another electron gun disclosed in U.S. Pat. No. 4,143,293, a first grid has a vertically elongated opening, and a sub electrode is disposed between second and third grids. The sub electrode has an asymmetric opening. The electron gun of this prior art example is used to form a vertically elongated beam spot at the central portion of the screen. However, the necessity of providing the sub electrode results in inconvenience.
In still another electron gun disclosed in Japanese Patent Disclosure No. 56-149755, cylindrical edges of openings of third and fourth grids are set at a predetermined length so as to form a vertically elongated beam spot at the central portion of the screen. In addition to these examples, in an electron gun described in U.S. Pat. No. 4,242,613, a first grid has an opening portion which comprises openings formed in a criss-cross manner at one side of the first grid which opposes a cathode and at the other side thereof which opposes a second grid. According to this construction, horizontal focusing of the electron beam differs from vertical focusing thereof by a given magnitude, thereby decreasing deflection defocusing.
In the conventional electron guns described above, deflection defocusing can be decreased by forming a vertically elongated beam spot at the central portion of the screen. However, resolution at the central portion of the screen is thereby degraded. More particularly, since the beam spot has a vertically elongated shape, the width of the horizontal line on the screen is increased when the electron beam is deflected along the horizontal axis.
In addition to these conventional electron guns, an electron gun is described in Japanese Patent Disclosure No. 54-150961. In this electron gun, two sub electrodes are added to perform dynamic focusing. However, a separate power supply is required, resulting in a large and complex construction.